Method and system for harmonizing an operation area for a mobile device in a cellular radio network

ABSTRACT

The invention relates to a method and a system for harmonising an operational area for a mobile device on a cell-by-cell-basis in a cellular radio network, like a GSM network, wherein, in idle mode, the mobile device compares field strength levels of different base stations with each other in order to set up a power budget, and selects the most appropriate base station in accordance with said power budget, wherein for prioritizing a predetermined base station an individual additional offset term is applied in the power budget. The invention is characterized in that in connection mode, the mobile device compares the power received from different base stations with each other in order to set up a power budget, and selects the most appropriate base station in accordance with said power budget, wherein for prioritizing a predetermined base station an individual additional offset term is applied in the power budget, and both offset terms are set so as to be similar to each other or the same.

FIELD OF THE INVENTION

The present invention relates to a method and system for harmonizing anoperational area for a mobile device on a cell-by-cell-basis in acellular radio network, like a GSM (Global System of Mobilecommunication) network.

BACKGROUND OF THE INVENTION

In current systems, the flexibility resulting from the existing largeset of parameters included in the different algorithms cannot be fullyused because of its complexity. In the planning stage, homogeneousnetworks are normally considered, as the large set of parameter makesthe detailed planning process on a cell-by-cell basis a time-consumingtask. As a consequence, the operators fix parameters to a common set ofdefault values shared between cells, even if no optimum performance interms of quality/capacity is reached. This homogeneity hypothesis may befar from reality, where interference or propagation severity can varyboth in time and space over the network.

Moreover, a few operators extend the parameter optimisation byclassifying the cells in accordance with certain scenarios like rural,urban, tunnel, indoors etc. and/or in accordance with the layer/bandused (like Macro900/1800, Micro900/1800, Pico1800, Motorway900). So, thecells are divided into scenario groups or layer/band groups, and commondefault parameter values are shared which, however, are not optimum.

In those cases where new features are enabled, so-called field trialsare required. During the tuning process, conclusions from parameterchanges are difficult to derive, and final settings are nearly always onthe safe side with its limited results. Moreover, such trials arenormally focused on global parameters of features under study, andparameter optimisation of adjacent cells is hardly ever done. So,differences between adjacent cells are rarely considered due to a higheffort required. Therefore, the potential of so-called adjacencyparameters is not fully exploited.

A final limited parameter tuning based on cell/area level performanceindicators is normally carried out only over those cells whereperformance problems are existing.

Even if an optimum value were reached by means of the above-mentionedtrials, changes in traffic or environment conditions, like theinstallation of new cells, changes of interference level by frequencyre-planning etc., would force a further re-tuning process of theparameter base, where no automatic reactive process is currently in use.Such a situation could be analysed as a result of slow trends, like thechange of the number of user registrations, or fast changes, e.g. of thenumber of connections, during a short time period, like an hour or aday.

The obvious conclusion is the inability to grasp the full flexibility ofthe wide set of parameters.

In particular, parameters defining the cell operational area during theidle (camping) mode and the connection mode are not synchronised. Infact, cell attractiveness during connection mode may be completelydifferent from idle mode due to traffic management strategies, causingunnecessary flow of users. The final result will be waste of bandwith insignalling and risk of dropped calls during the handover process.

From this analysis, it is obvious that unnecessary handover may beavoided if users camp on the cell which they are more likely to end-upin. Doing so, a great potential performance gain may be achieved.Moreover, operators may benefit from an automatic individual (i.e. cellbased) optimising and tuning process. This would help operators in thetuning process and offer cost savings and improved performance, despitenetwork inhomogeneities both in space (i.e. cell) and time (e.g. day orhour).

SUMMARY OF THE INVENTION

It is an object of the present invention to propose a method and asystem providing an automatic tuning process for harmonizing theoperational area in every mode on a cell-by-cell-basis.

In order to achieve above and further objects, according to a firstaspect of the present invention, there is provided a method forharmonising an operational area for a mobile device on acell-by-cell-basis in a cellular radio network, like a GSM network,

wherein, in idle mode, the mobile device compares field strength levelsof different base stations with each other in order to set up a powerbudget, and selects the most appropriate base station in accordance withsaid power budget, wherein for prioritizing a predetermined base stationan individual additional offset term is applied in the power budget;characterized in that,in connection mode, the mobile device compares the power received fromdifferent base stations with each other in order to set up a powerbudget, and selects the most appropriate base station in accordance withsaid power budget, wherein for prioritizing a predetermined base stationan individual additional offset term is applied in the power budget, andboth offset terms are set so as to be similar to each other or the same.

According to a second aspect of the present invention, there is provideda system for harmonising an operational area for a mobile device on acell-by-cell-basis in a cellular radio network, like a GSM network,comprising

idle mode means for comparing field strength levels of different basestations with each other in order to set up a power budget and selectingthe most appropriate base station for a mobile device in accordance withsaid power budget, wherein for prioritizing a predetermined base stationan individual additional offset term is applied in the power budget;characterized by,connection mode means for comparing the power received from differentbase stations with each other in order to set up a power budget andselecting the most appropriate base station for the mobile device inaccordance with said power budget, wherein for prioritizing apredetermined base station an individual additional offset term isapplied in the power budget, andboth offset terms are set so as to be similar to each other or the same.

The mobile device in idle mode compares field strength levels comingfrom different cells with each other and selects the best one from them.In this power budget, an individual additional offset may be applied ifone cell must be prioritised for artificially increasing itsattractiveness.

In dedicated or connection mode, the mechanism which assures that themobile device is always within the cell offering the best coverage isassured by handover mechanisms. Similar to idle mode case, in accordancewith the present invention, received power is compared and the best basestation is chosen through power budget. Priority biasing may as well beapplied to (un)favour users into one specific cell by means of margins,which may be the case for reactive load balancing mechanisms.

In accordance with the present invention, both offset terms are similarto each other or the same, in order to maintain a coherent power budgetevaluation in order to prioritise cells.

Preferably, the offset terms are so-called biasing margin terms.

Usually, idle mode parameters relate to the size of cells of adjacentbase stations, wherein the idle mode parameters may affect to the sizeof cells of adjacent base stations in a non-directional way. In contrastthereto, connection mode parameters may take into account differencesbetween cells of adjacent base stations.

The main difficulty may reside in the above mentioned fact thatparameters related to idle mode or user camping behaviour arecell-specific, whereas parameters related to connection mode or handovermechanisms may be adjacency-specific. So, some sort of average processof the operational area in handovers may be needed in order to draw aglobal conclusion for cell selection/reselection parameters. Once thisaverage margin term in handovers for a cell with its adjacencies hasbeen reckoned, the biasing term for idle mode will be set equal inaccordance with the present invention. Further considerations related toadjacencies strengths (e.g. handover share from specific adjacency) mayas well be accounted for through weighting operations in such averagingprocess.

The power budget PBGT_(serv)(adj) is calculated by using the equationPBGT _(serv)(adj)=RxLev _(adj) −RxLev _(serv) −HoMarginPBGT_(serv)(adj),where RxLev_(adj) is the received downlink level from an adjacent basestation, RxLev_(serv) is the received downlink level from the currentlyserving base station, andHoMarginPBGT_(serv)(adj) is a handover margin power budget parameter toartificially change priority of the adjacent base station for handoverfrom the serving base station; andthe handover margin power budget parameter may be averaged by using theequation

${{\overset{\_}{HoMarginPBGT}}_{adj}\lbrack{dB}\rbrack} = \frac{{{HoMarginPBGT}_{serv}({adj})} + {{HoMarginPBGT}_{adj}({serv})}}{2}$where HoMarginPBGT_(adj)(serv) is a handover margin power budgetparameter to artificially change priority of the currently serving basestation for handover from the adjacent base station.

The priority difference Adj PriorDifference_(adj) [dB] between the cellof the currently serving base station and the cell of an adjacent basestation may be calculated by using the equation

${{AdjPriorDifference}_{adj}\lbrack{dB}\rbrack} = {\left( {{{HoMarginPBGT}_{serv}({adj})} - \;\overset{\_}{HoMarginPBGT}} \right) = {\left( {{{HoMarginPBGT}_{adj}({serv})} - \;\overset{\_}{HoMarginPBGT}} \right).}}$

In a preferred embodiment wherein in connection mode for prioritizing aplurality of base stations a corresponding plurality of individualadditional offset terms are provided, the offset terms are averaged soas to create an averaged offset term.

Preferably, the averaged offset term X is calculated by using theequation

${X = {\underset{adj}{Average}\left\lbrack {\left( \frac{{Incom} + {{Outg}.{HOTraffic}_{{adj}\leftrightarrow{serv}}}}{{\sum\limits_{adj}^{\;}{Incom}} + {{Outg}.{HOTraffic}_{{adj}\leftrightarrow{serv}}}} \right) \cdot {AdjPriorDifference}_{adj}} \right\rbrack}},$where Incom+Outg.HOTraffic_(adj⇄serv) reflects the sum of incoming andoutgoing handovers between adjacent and serving cell.

Moreover, a presently preferred embodiment of the invention, wherein inidle mode a basis C2 cell reselection is calculated by using theequationC2_(serv)=(RxLev _(serv) −RxLevAccesMin _(serv))+CellReselectOffset_(serv)where RxLev_(serv) is the received downlink level from the currentlyserving base station,

RxLevelAccessMin_(serv) is the minimum downlink level threshold receivedby the mobile device to camp in a cell, and

CellReselectOffset_(serv) is the biasing term to artificially change thepriority of the currently serving base station,

is characterized in that if the averaged offset term is positive, it istaken as the parameter CellReselectOffset_(serv).

So, in accordance with the present invention, differences in operationalareas between idle and connected modes that produce inefficient flow ofusers are essentially overcome.

A further advantage of the present invention is that traffic andenvironment changes are tracked by means of automatic parameterauto-tuning in order to achieve the best performance without userinteraction. As a consequence, less parameters are required to beadjusted. Further, unnecessary flow of user between cells is avoidedresulting in saving bandwidth in terms of signalling and achieving abetter efficiency. Moreover, the present invention provides for a betterperformance since unnecessary handovers are reduced and every handovermeans a risk for a dropped call. Finally, a fast optimisation capabilityis achieved by the present invention since automatic (i.e. non-user)control enables the network for fast auto-tuning algorithms, wheneveragile changes are allowed.

The present invention can be implemented in any kind of cellular mobilenetwork systems like e.g. GSM or UMTS (Universal mobiletelecommunications system).

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention will be described in greaterdetail based on a preferred embodiment with reference to theaccompanying drawings wherein

FIG. 1 shows unnecessary flow of users;

FIG. 2 shows traffic management in connected mode through power budgetmargins; and

FIG. 3 shows idle mode and connected mode parameter influences on cellsize and shape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an unnecessary flow of users, which may be prevented. Acamped cell may differ from the final target cell, due to the fact thatidle mode and connected mode traffic management strategies (defined bytheir parameters) may be quite different.

So, a search for common metrics to prioritise cells, regardless thestage of the relationship between the user and the network, defines thekey process.

In GSM systems, C1 and C2 comparisons are the basic metrics used for theprioritisation of cells in idle mode, where the cell with a higher valuewill finally be chosen. Basic C2 cell reselection is calculated byequation:C2_(serv)=(RxLev _(serv) −RxLevAccesMin _(serv))+CellReselectOffset_(serv)  (1)where RxLev_(serv) is the received downlink level from beacon frequencyof the currently serving base station,

RxLevelAccessMin_(serv) is the minimum downlink level threshold receivedby the mobile device to camp in a cell, and

CellReselectOffset_(serv) is the biasing term to artificially change thepriority of the currently serving base station.

In connected mode, power budget equation is the metric used by thenetwork algorithm to protease cells in the target cell evaluationmechanism. Power budget follows the equation:PBGT _(serv)(adj)=RxLev _(adj) −RxLev _(serv) −HoMarginPBGT_(serv)(adj),  (2)where RxLeV_(adj/serv) is the received downlink level fromadjacent/serving cell beacon frequency, and

HoMarginPBGT_(serv)(adj) is the handover margin power budget parameterterm to artificially change cell priority of adjacent cell.

Multiple objectives can be achieved with the last handover margin powerbudget term. First, certain hysteresis degree is assured by means ofsymmetrical values so that instabilities in the form of the so-calledping-pong handovers are prevented. Second, traffic management strategiesare normally based on the asymmetrical modification of this margin term,as it is shown in FIG. 2. This figure shows how sign and magnitude ofmargins (un)constrain the flow of users in different directions, thusaffecting operation areas in a directional way. Aligned to this idea,congestion relief (or load balancing) algorithms benefit from theinstantaneous ability to modify margins, temporarily (un)constrainingthe flow of users between cells.

Due to the fact that margins must ensure hysteresis, margins will not bezero, even when no biasing is applied. This observation shows that themargin term will include a symmetrical component for hysteresis andasymmetrical component in order to re-direct traffic. This last termwill show the biasing into any direction.

It is important to note that CellReselectOffset as given in the aboveequation (1) is a cell parameter, thus affecting adjacencies in anon-directional way. On the other side, HoMarginPBGT_(serv)(adj) asgiven in the above equation (2) is an adjacent cell parameter, whichenables it to have influence not only on the size, but also on the shapeof the cell. In other words, idle mode parameters affect the size ofadjacencies in a non-directional way, while handover algorithms may takeinto account differences between adjacencies.

This idea is presented in FIG. 3, where different shape and size isobtained, depending on the selection of the parameters, which are tuned.In particular, this figure shows that adjacent cell parameters inconnected mode define the shape of the cell through differentprioritisation biasing and that cell parameters affect adjacencyoperational area in a global way. Based on this fact, some sort ofaverage process of the operational area in handovers is needed in orderto draw a global conclusion for cell reselection parameters. Unlessmargins are the same for every adjacency, operational areas will notperfectly match: minimum error between operational areas is the finaltarget.

The automatic tuning process in charge of the CellReselectOffsetparameter tuning will run the following sequence of operations on a cellindividual basis:

-   1. For every adjacency, the symmetrical component of the margin must    be cancelled in order to calculate the deviation from the situation    where just raw power is the final target. Thus, the first step will    try to reckon the centre of the hysteresis region.    -   For this purpose, the handover margin power budget parameter is        averaged by using the equation

$\begin{matrix}{{{\overset{\_}{HoMarginPBGT}}_{adj}\lbrack{dB}\rbrack} = \frac{{{HoMarginPBGT}_{serv}({adj})} + {{HoMarginPBGT}_{adj}({serv})}}{2}} & (3)\end{matrix}$

-   -   where HoMarginPBGT_(adj)(serv) is a handover margin power budget        parameter to artificially change priority of the currently        serving base station.    -   So, the process depicted in formula 1 maps two values (incoming        and outgoing direction of the adjacency) into a unique value for        the centre, assigned for the whole adjacency.

-   2. Once centre hysteresis point has been defined, the deviation from    symmetrical situation Adj Prior Difference may easily be calculated    from the equation

$\begin{matrix}{{{AdjPriorDifference}_{adj}\lbrack{dB}\rbrack} = {\left( {{{HoMarginPBGT}_{serv}({adj})} - \;\overset{\_}{HoMarginPBGT}} \right) = \left( {{{HoMarginPBGT}_{adj}({serv})} - \;\overset{\_}{HoMarginPBGT}} \right)}} & (4)\end{matrix}$

-   -   The obtained result shows the asymmetrical component of the        margins, averaging both directions of the adjacency (i.e.        incoming and outgoing handovers).

-   3. As stated before, connected mode parameters may modify the cell    area in a non-directional way. So, an averaging process is needed so    that a final conclusion must be extracted from the set of    adjacencies. Those adjacencies with a higher number of handovers    must be weighted favourably so that the cell size is adapted to the    real operational area. This averaging process is seen by the    equation

$\begin{matrix}{{{CellReselectOffset}_{serv}\lbrack{dB}\rbrack} = {{\underset{adj}{Average}\left\lbrack {\left( \frac{{Incom} + {{Outg}.{HOTraffic}_{{adj}\leftrightarrow{serv}}}}{{\sum\limits_{adj}^{\;}{Incom}} + {{Outg}.{HOTraffic}_{{adj}\leftrightarrow{serv}}}} \right) \cdot {AdjPriorDifference}_{adj}} \right\rbrack}.}} & (5)\end{matrix}$

-   -   where Incom+Outg.HOTraffic_(adj⇄cell) reflects the sum of        incoming and outgoing handovers between adjacent and serving        cells.

Finally, priority bias is forced positive, i.e.CellReselectOffset _(serv) [dB]=Max(CellReselectOffset, 0)  (6)

as it is a constraint for the CellReselectOffset parameter.

When a cell is user receptor, the value must be positive. On thecontrary, if a cell is refusing users, any adjacency will tend to show ahigh priority positive bias (averaged with their own whole set ofadjacencies). In this way, double counting is prevented

Although the invention is described above with reference to an exampleshown in the attached drawings, it is apparent that the invention is notrestricted to it, but can vary in many ways within the scope disclosedin the attached claims.

1. A method, comprising: in idle mode, comparing, by a mobile device,field strength levels of different base stations with each other inorder to set up a power budget, and selecting the most appropriate basestation in accordance with said power budget, wherein for prioritizing apredetermined base station an individual additional offset term isapplied in the power budget, the method being for harmonizing anoperational area for a mobile device on a cell-by-cell-basis in acellular radio network; and in connection mode, comparing, by the mobiledevice, the power received from different base stations with each otherin order to set up a power budget, and selecting the most appropriatebase station in accordance with said power budget, wherein forprioritizing a predetermined base station an individual additionaloffset term is applied in the power budget, wherein both offset termsare set so as to be similar to each other or the same.
 2. The methodaccording to claim 1, wherein idle mode parameters relate to the size ofcells of adjacent base stations.
 3. The method according to claim 2,wherein the idle mode parameters affect to the size of cells of adjacentbase stations in a non-directional way.
 4. The method according to claim1, wherein connection mode parameters take into account differencesbetween cells of adjacent base stations.
 5. The method according toclaim 1, wherein said offset terms are biasing margin terms.
 6. Themethod according to claim 5, wherein the power budget PBGT_(serv)(adj)is calculated by using the equationPBGT _(serv)(adj)=RxLev _(adj) −RxLev _(serv) −HoMarginPBGT_(serv)(adj), where RxLev_(adj) is the received downlink level from anadjacent base station, RxLev_(serv) is the received downlink level fromthe currently serving base station, and HoMarginPBGT_(serv)(adj) is ahandover margin power budget parameter to artificially change priorityof the adjacent base station for handover from the serving base station;characterized in that the handover margin power budget parameter isaveraged by using the equation${{\overset{\_}{HoMarginPBGT}}_{adj}\lbrack{dB}\rbrack} = \frac{{{HoMarginPBGT}_{serv}({adj})} + {{HoMarginPBGT}_{adj}({serv})}}{2}$where HoMarginPBGT_(adi)(serv) is a handover margin power budgetparameter to artificially change priority of the currently serving basestation.
 7. The method according to claim 6, characterized in that thepriority difference Adj PriorDifference_(adj) between the cell of thecurrently serving base station and the cell of an adjacent base stationis calculated by using the equation${{AdjPriorDifference}_{adj}\lbrack{dB}\rbrack} = {\left( {{{HoMarginPBGT}_{serv}({adj})} - \;\overset{\_}{HoMarginPBGT}} \right) = {\left( {{{HoMarginPBGT}_{adj}({serv})} - \;\overset{\_}{HoMarginPBGT}} \right).}}$8. The method according to claim 7, characterized in that the averagedoffset term X is calculated by using the equation${X = {\underset{adj}{Average}\left\lbrack {\left( \frac{{Incom} + {{Outg}.{HOTraffic}_{{adj}\leftrightarrow{serv}}}}{{\sum\limits_{adj}^{\;}{Incom}} + {{Outg}.{HOTraffic}_{{adj}\leftrightarrow{serv}}}} \right) \cdot {AdjPriorDifference}_{adj}} \right\rbrack}},$wherein Incom+Outg.HOTraffic_(adj⇄cell) reflects the sum of incoming andoutgoing handovers between adjacent and serving cells.
 9. The methodaccording to claim 1, characterized in that in connection mode theadditional offset term is created by averaging terms for both directionsbetween a cell of a currently serving base station and a cell of anadjacent base station.
 10. The method according to claim 1, wherein inconnection mode for prioritizing a plurality of base stations acorresponding plurality of individual additional offset terms areprovided; characterized in that the offset terms are averaged so as tocreate an averaged offset term.
 11. The method according to claim 10,wherein in idle mode a basis C2 cell reselection is calculated by usingthe equationC2_(serv)=(RxLev _(serv) −RxLevAccesMin_(serv))+CellReselectOffset_(serv), where RxLev_(serv) is the receiveddownlink level from the currently serving base station,RxLevAccessMin_(serv) is the minimum downlink level threshold receivedby the mobile device to camp in, a cell, and CellReselectOffset_(serv)is the biasing term to artificially change the priority of the currentlyserving base station; characterized in that, if the averaged offset termis positive, it is taken as the parameter CellReselectOffset_(serv). 12.A system, comprising: an idle mode unit configured to compare fieldstrength levels of different base stations with each other in order toset up a power budget and to select the most appropriate base stationfor a mobile device in accordance with said power budget, wherein forprioritizing a predetermined base station an individual additionaloffset term is applied in the power budget, the system being used forharmonizing an operational area for a mobile device on acell-by-cell-basis in a cellular radio network; and a connection modeunit configured to compare the power received from different basestations with each other in order to set up a power budget, and toselect the most appropriate base station for the mobile device inaccordance with said power budget, wherein for prioritizing apredetermined base station an individual additional offset term isapplied in the power budget, wherein both offset terms are set so as tobe similar to each other or the same.
 13. The system according to claim12, wherein idle mode parameters relate to the size of cells of adjacentbase stations.
 14. The system according to claim 13, wherein the idlemode parameters affect to the size of cells of adjacent base stations ina non-directional way.
 15. The system according to claim 12, whereinconnection mode parameters take into account differences between cellsof adjacent base stations.
 16. The system according to claim 12, whereinsaid offset terms are biasing margin terms.
 17. The system according toclaim 16, wherein the power budget PBGT_(serv)(adj) is calculated byusing the equationPBGT _(serv)(adj)=RxLev _(adj) −RxLev _(serv) −HoMarginPBGT_(serv)(adj), where RxLev_(adj) is the received downlink level from anadjacent base station, RxLev_(serv) is the received downlink level fromthe currently serving base station, and HoMarginPBGT_(serv)(adj) is ahandover margin power budget parameter to artificially change priorityof the adjacent base station for handover from the serving base station;characterized in that said averaging means averages the handover marginpower budget parameter by using the equation${{\overset{\_}{HoMarginPBGT}}_{adj}\lbrack{dB}\rbrack} = \frac{{{HoMarginPBHGT}_{serv}({adj})} + {{HoMarginPBGT}_{adj}({serv})}}{2}$where HoMarginPBGT_(adi)(serv) is a handover margin power budgetparameter to artificially change priority of the currently serving basestation.
 18. The system according to claim 17, characterized by meansfor calculating the priority difference Adj PriorDifference_(adi)between the cell of the currently serving base station and the cell ofan adjacent base station by using the equation${{AdjPrior}\;{{Difference}_{adj}\lbrack{dB}\rbrack}} = {\left( {{{HoMarginPBGT}_{serv}({adj})} - \overset{\_}{HoMarginPBGT}} \right) = {\left( {{{HoMarginPBGT}_{adj}({serv})} - \overset{\_}{HoMarginPBGT}} \right).}}$19. The system according to claim 18, characterized in that saidaveraging means calculates the averaged offset term X by using theequation${X = {\underset{adj}{Average}\left\lbrack {{\left( \frac{{Incom} + {{Outg}.{HOTraffic}_{{adj}\leftrightarrow{serv}}}}{{\sum\limits_{adj}{Incom}} + {{Outg}.{HOTraffic}_{{adj}\leftrightarrow{serv}}}} \right) \cdot {AdjPrior}}\;{Difference}_{adj}} \right\rbrack}},$wherein Incom+Outg.HOTraffic_(adj⇄cell) reflects the sum of incoming andoutgoing handovers between adjacent and serving cells.
 20. The systemaccording to claim 12, characterized by an averaging unit for averagingterms for both directions between a cell of a currently serving basestation and a cell of an adjacent base station in the connection mode.21. The system according to claim 12, wherein in connection mode forprioritizing a plurality of base stations a corresponding plurality ofindividual additional offset terms are provided; characterized byaveraging means for averaging said offset terms so as to create anaveraged offset term.
 22. The system according to claim 21, wherein saididle mode means calculates a basis C2 cell reselection by using theequationC2_(serv)=(RxLev_(ser) −RxLevAccesMin_(serv))+CellReselectOffset_(serv), where RxLev_(serv) is the receiveddownlink level from the currently serving base station,RxLevAccesMin_(serv) is the minimum downlink level threshold received bythe mobile device to camp in a cell, and CellReselectOffset_(serv), isthe biasing term to artificially change the priority of the currentlyserving base station; characterized by means for taking the averagedoffset term as the parameter CellReselectOffset_(serv) if the averagedoffset term is positive.
 23. An apparatus, comprising: idle mode meansfor comparing field strength levels of different base stations with eachother in order to set up a power budget and selecting the mostappropriate base station for a mobile device in accordance with saidpower budget, wherein for prioritizing a predetermined base station anindividual additional offset term is applied in the power budget; andconnection mode means for comparing the power received from differentbase stations with each other in order to set up a power budget, andselecting the most appropriate base station for the mobile device inaccordance with said power budget, wherein for prioritizing apredetermined base station an individual additional offset term isapplied in the power budget, wherein both offset terms are set so as tobe similar to each other or the same.